docs: remove invalid async option from docs (fix #1769) (#1796)

src/Clone.js

@@ -184,7 +184,6 @@ export class DocumentCloner {
                     return this.renderer(
                         documentElement,
                         {
-                            async: this.options.async,
                             allowTaint: this.options.allowTaint,
                             backgroundColor: '#ffffff',
                             canvas: null,

src/index.js

@@ -8,7 +8,6 @@ import Logger from './Logger';
 import {renderElement} from './Window';
 
 export type Options = {
-    async: ?boolean,
     allowTaint: ?boolean,
     backgroundColor: string,
     canvas: ?HTMLCanvasElement,
@@ -50,7 +49,6 @@ const html2canvas = (element: HTMLElement, conf: ?Options): Promise<*> => {
     const defaultView = ownerDocument.defaultView;
 
     const defaultOptions = {
-        async: true,
         allowTaint: false,
         backgroundColor: '#ffffff',
         imageTimeout: 15000,